Boxer style crankshafts ?

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Bille Floyd

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Always wondered about this :
In a boxer style twin , like the 32Hp 3W-342 ; why are the pistons , and
connection rods , staggered slightly and use two bearings
on the crankshaft ? Why can ya just use one bearing on the crankshaft
and let the postons and connecting rods counterbalance each other ?
Couldn't ya get rid of the counterbalance weights on the crankshaft, that
way ?


BTW -- there use to be a company that made a reduction for that
engine ; for use on ultralights !

Bille

3W-crankshaft_342i.jpg
 

TFF

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Not quite apples but Harley vs BMW motorcycle.

Pros for one crank pin. Easier to make at the manufacturing level. Less materials. Cons balance and firing order. I have a Saito 90T RC engine that is one pin opposed twin engine. It’s not as smooth as the later boxer version or the OS Geminis. I think the odd firing degrees is 180/540 not an even 360 like a boxer. The engine is like a Harley has that bang bang skip cadence. Even, boxer is bang bang even beat.

The weight of the pistons and rods do the same thing. The inertia is not the same for each side because geometry has the moving weight dwell differently. Not as good a balance. Crank Balance weights are purely engineering and how smooth you want the engine to run and how long the crank needs to survive.

Boxer will weigh more all else equal.

It’s really down to how much money was put into development and how cheap you need to sell it and how much life and performance you need to get. It’s not either or.
 

Bill-Higdon

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Not quite apples but Harley vs BMW motorcycle.

Pros for one crank pin. Easier to make at the manufacturing level. Less materials. Cons balance and firing order. I have a Saito 90T RC engine that is one pin opposed twin engine. It’s not as smooth as the later boxer version or the OS Geminis. I think the odd firing degrees is 180/540 not an even 360 like a boxer. The engine is like a Harley has that bang bang skip cadence. Even, boxer is bang bang even beat.

The weight of the pistons and rods do the same thing. The inertia is not the same for each side because geometry has the moving weight dwell differently. Not as good a balance. Crank Balance weights are purely engineering and how smooth you want the engine to run and how long the crank needs to survive.

Boxer will weigh more all else equal.

It’s really down to how much money was put into development and how cheap you need to sell it and how much life and performance you need to get. It’s not either or.
Also the WW1 Penguin trainer engine made by Lawrence was a 1 crank pin engine they were know for shaking things apart
 

n3puppy

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Also the WW1 Penguin trainer engine made by Lawrence was a 1 crank pin engine they were know for shaking things apart
Yup - vibration on a single pin twin is going to be an issue since the mass of both pistons/rods is always going in the same direction rocking the engine. Plus the two individual firing impulses are helping that rock from side to side.

Can get away with it on single pin four cylinder engines (flat V) because the two sets of pistons are moving in different directions off-setting each other.

F59C1EF9-2DD5-4124-9864-0EA65DBFBEE9.jpeg E61F0927-1156-49D8-B4BE-54BDA2D95806.jpeg
 
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wsimpso1

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Two, Four, and Six cylinder opposed engines really ought to be Boxers - opposed pins for each cylinder pair) to keep the engine from shaking itself loose and destroying the prop while it is at it. And they usually are. Go to eights and twelves (think Porsche 908 and 917) and a boxer crank becomes really long cranks, and really heavy by the time you make it stiff enough for the high firing frequencies we get with an even fire twin. The solution is to use one crank pin for each pair of cylinders. Crankshaft gets shorter and stiffer, works great.

Then there is the whole issue of two strokes. If it is crankcase pumped, I believe each cylinder needs its own chamber and crank pin. If you make it breath by using a blower and turbocharger, who cares?

Billski
 

JP Straley

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Take a look at the Hexadyne P-60. A flat twin set up somewhat like a Harley. HD cylinders are not offset, they use a forked rod on a common crankpin. The Hex is quite different, uses two small rods flanking the opposite cylinder. Two crankpins. Note that modern vertical twins (say, Triumph) set the crankings at 90 degrees apart. Better control of vibration...pretty good engineering solution.
 

sotaro

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[/QUOTE]
Then there is the whole issue of two strokes. If it is crankcase pumped, I believe each cylinder needs its own chamber and crank pin.
Usually a crankcase compression two stroke needs it's own crankcase chamber to compress the air fuel for transfer to the combustion chamber. If a boxer, 2 cylinders can share a crankcase. With modern porting, the need for crankcase stuffing is reduced, especially with two pistons in one crankcase.

Bill, I assume a boxer twin two stroke vibrates less than a single two stroke of the same total displacement. How would the torsionally resonances compare?
 

wsimpso1

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917s took the power out of the center.
Yes. It is all about getting it to work and live at low weight. End drive would work but required bigger crank sections, as would a Boxer scheme,and would drive longer connecting rods, bigger cases, wider engine, more weight. Going common pins allowed a lighter crank and more compact lighter engine total package. Going center drive allowed lower weight too. Crankshaft was titanium alloy too. They were not shy about cost, but bulk/weight had to be kept down almost as much as in spacecraft.

With 12 cylinders a huge fraction of the yaw axis vibe from common pin (instead of Boxer crank scheme) can be canceled by making the throw sets mirror image from center, so the vibe need for Boxer scheme was removed too.

One other neat feature about center PTO is it puts a long springy shaft in the system. In an even fire 12, that shaft all by itself can isolate 6x (firing) and 12x (twice firing) vibe from the rest of the powertrain. Gears at the crank will still sing like mad... Good thing racers and their fans view this as music.

Billski
 

PMD

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A boxer 2 cycle twin has similar torsional characteristics to a similar displacement 2 stroke single. Can be dealt with, but needs some serious energy storage capacity in a soft system. The Hexadyne (modified Weslake design) is a four cycle, so works better torsionally with same frequency but longer duration thus lower amplitude of power pulse but is noteworthy as to just how large the cyclic rocking couple is for offset journals.
 

PMD

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One other neat feature about center PTO is it puts a long springy shaft in the system. In an even fire 12, that shaft all by itself can isolate 6x (firing) and 12x (twice firing) vibe from the rest of the powertrain. Gears at the crank will still sing like mad... Good thing racers and their fans view this as music.
One of the great mysteries in life to me is why nobody made a transverse V12 from a pair of back-to-back 60 degree V6 engines. Spur gear connection at center of crank could then go right into dog box that also could be center fed with half gearset on each side and final drive coming off center of case from middle of countershaft(s). Yes, very custom gearbox but could use really one off the shelf engine and one reverse rotation such as the 3.5 GM. also cloud work with bevel gear coupling feeding normal longitudinal transaxle IF the rear main could handle thrust loads.








i
 

rmeyers

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One of the great mysteries in life to me is why nobody made a transverse V12 from a pair of back-to-back 60 degree V6 engines. Spur gear connection at center of crank could then go right into dog box that also could be center fed with half gearset on each side and final drive coming off center of case from middle of countershaft(s). Yes, very custom gearbox but could use really one off the shelf engine and one reverse rotation such as the 3.5 GM. also cloud work with bevel gear coupling feeding normal longitudinal transaxle IF the rear main could handle thrust loads.
Detroit Diesel 12V-92. Two 6V-92 engines joined. I have heard, but cannot verify, that they were available with center drive. I do remember an instructor at diesel school saying that they were produced with center drive for mining equipment.
 

wsimpso1

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One of the great mysteries in life to me is why nobody made a transverse V12 from a pair of back-to-back 60 degree V6 engines. Spur gear connection at center of crank could then go ...

Biggest reason not to in road vehicles is just that it is NOISEY. The various firing vibe orders open and the close the gear tooth contacts, with impact, at low rpm. It is tolerated in motorcycles and racing equipment, but you would never tolerate it in your everyday drivers. The various impact modes also drive up the gear sizes. Once you get the other side of the vibe isolator, your gearsets get a lot quieter...

Couldn't you vibe isolate the crank gear? You might go crazy trying to maintain adequate conjugate action and alignment of the gear set. Gears stay quiet and live a long time only with really good tooth form and alignment... which might be impossible to do at any reasonable weight while the gear rotates on the crankshaft.

Billski
 

Bille Floyd

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Would the rotating counterweight in a Radial engine ; be
lighter or heavier per cylinder ; than the counterweights
per cylinder, on the crankshaft for an inline 6 ?

I realize radial engines are usually odd number of
cylinders, and inline engines are usually even ; just
trying to see the average weight dispersal for the
counterweights.

Bille
 
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JP Straley

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One of the great mysteries in life to me is why nobody made a transverse V12 from a pair of back-to-back 60 degree V6 engines. Spur gear connection at center of crank could then go right into dog box that also could be center fed with half gearset on each side and final drive coming off center of case from middle of countershaft(s). Yes, very custom gearbox but could use really one off the shelf engine and one reverse rotation such as the 3.5 GM. also cloud work with bevel gear coupling feeding normal longitudinal transaxle IF the rear main could handle thrust loads.








i
Hey, PMD, your mystery is now solved. In the 1960s GMC put two V6 engines together end-to-end. 702 cubic inches, almost 1500 lb in running order. Of interest to the corvair tribe is their distributor setup. Take a look. Here's the link: 702ci Thunder V12 GMC - Car Craft Magazine

JPS
 

PMD

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Hey, PMD, your mystery is now solved. In the 1960s GMC put two V6 engines together end-to-end. 702 cubic inches, almost 1500 lb in running order. Of interest to the corvair tribe is their distributor setup. Take a look. Here's the link: 702ci Thunder V12 GMC - Car Craft Magazine

JPS
Familiar with that engine (we had them in some semis and big straight trucks in the days where gassers were still used in big trucks, but these took power off of the end of one crank, not split off from the center.

BTW: the 12V Detroit diesels were NOT "compound" engines, they were dedicated V12 blocks using a pair of 6-71 heads. The 16v and 24v engines were indeed a pair of V8s and a pair of V12s simply bolted together using 4x 4 cyl heads and 4 x 6 cyl heads. These things were the ultimate in building block principal design and many parts were shared between whole families of similar cylinder displacements (53, 71, 92 and 149 IIRC). I have a 4-53 and also a 6V92T and the latter REALLY rocks and rolls.
 
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